It is possible this is to stop misrouted messages, however, they didn't add it in response to the recent problems. There are some Google hits from 2007-2008 whe

It is possible this is to stop misrouted messages, however, they didn't add it in response to the recent problems. There are some Google hits from 2007-2008 where the field was present, for example https://developer.pidgin.im/ticket/3360#comment:15 https://developer.pidgin.im/ticket/3360#comment:15. Before ~2009-2010 the field seems to have been 8 hex encoded bytes (half a MD5 hash?), after that they switched to the base64 format.